<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <div id="parent">
        <div id="child">
        </div>
    </div>


</body>
<style>

    #parent {
        display:flex;
        width:300px;
        height:300px;
        outline:solid 1px;
        justify-content:center;
        align-content:center;
        align-items:center;
    }
    #child {
        width:100px;
        height:100px;
        outline:solid 1px;
    }

</style>
</html>